Display systems for digital television receivers, including high definition television receivers, and computer or multi-media monitors may have one or more modes with horizontal scanning rates higher than (e.g., 2.14 times) the conventional 1H scanning rate in accordance with the conventional NTSC, PAL and SECAM video transmission standards. In such higher than normal scanning rate monitor/receiver display systems, an automatic kinescope bias (AKB) arrangement has a shorter cathode cut-off current measurement time period compared with that of display systems operating in accordance with the conventional standards. It is herein recognized that prior AKB arrangements may not operate properly in higher than normal scanning rate display systems for the following reasons.
AKB systems employ feedback to regulate the beam cut-off level of kinescopes. Typically, an AKB system generates beam cut-off test pulses that are applied to a kinescope cathode. One horizontal line (typically a line of the vertical interval) is used to measure the beam cut-off current of each gun of the color kinescope (or CRT). A cathode current sensor detects the actual beam cut-off current produced in response to the test pulse and produces a voltage pulse across a load resistance. The measurement voltage pulse across the load resistance is compared to a reference level during the measurement time period. The bias voltage for each CRT gun is then adjusted by the system to correct for errors in cut-off bias voltages.
The process of turning on and turning off each gun causes the kinescope capacitance to be charged and discharged, The result is that the red, green and blue AKB measurement pulses may exhibit overshoots on each transition that may interfere with accurate measurement of the pulse amplitudes and therefore interfere with the adjustment of the cut-off bias voltages.
The presence of such overshoots for normal 1H operation (i.e., standard horizontal scan rates) is not a significant problem because the AKB pulse measurement period can be relatively long compared to the width of the overshoots and because a relatively large capacitor may be placed across the load resistance because of the slower scanning rate. However, the AKB measurement period at higher than normal 1H scanning rates has a relatively limited duration. It has been found that using relatively large capacitor across the load resistance will tend to reduce the amplitude of the shoots but will also cause undesirable tilt or slope on each pulse. It is herein recognized that such pulse distortions (tilt or slope) can result in AKB pulse measurements errors in such higher than normal scan rate systems. More specifically, a non-constant amplitude of the pulse, as it is being compared to a reference, can cause the AKB system to change bias during the AKB period and to not adjust (or "servo") to a consistent level. Additionally, there may be color temperature errors or inconsistencies between the various scanning modes.